1. Field of the Invention
This invention relates to excore detectors for neutron flux monitoring systems and more particularly, to an improved excore detector assembly comprising a moderator subassembly for capturing epithermal neutrons and converting same to thermal neutrons which can be sensed by the detector subassembly thereby to afford increased detection sensitivity, while maintaining and satisfying both structural and electrical connection and shielding requirements, space compatibility constraints, and seismic qualifications.
2. State of the Prior Art
Excore detectors for use in neutron flux monitoring systems associated with nuclear reactors are well known. Excore detectors are positioned closely adjacent to the exterior of the reactor vessel and are intended to respond to and detect the level of neutron flux leakage from the reactor core, which is not absorbed by surrounding structures and materials. By thus monitoring the neutron fluency or neutron flux level, the excore detector provides an indication of the current power level of the reactor. Such detectors, and their associated instrumentation systems, are well-known and are of differing types. Excore power level detectors serve to monitor the power level of the reactor continuously during normal operating conditions and thus during generation of electrical power, while other types function to provide an indication of the reactor status during start-up and shutdown conditions. During all of these conditions, appropriate alarm and reactor trip functions are to be generated by the instrumentation system upon the detection of abnormal neutron fluency levels relative to the normal power level status of the reactor in its corresponding different stages of operation.
In more recent years, reactor designs have been changed to implement various improvements in structure and operation of nuclear power facilities with a primary goal of plant life extension (PLEX). Further improvements include the development of low leakage fuel, which contributes to overall improved fuel management. These improvements have lead to a reduction in the nuclear fluency from a reactor vessel. Design modifications as well have occurred with respect to the mechanical structure of the core to afford improved structural integrity and seismic qualification, and to accommodate and/or compensate for various thermal conditions. These modifications and changes, while having salutary, beneficial results as to safety and efficiency of operation of the reactor vessel, have adversely impacted the performance of excore neutron flux monitoring systems, rendering the same less effective for providing an accurate indication of the power level within the reactor.
To compensate for this reduced level of sensitivity of the excore detectors and the resultant decreased effectiveness of the excore neutron flux monitoring systems, improvements have been implemented for increasing the response sensitivity of the associated monitoring system electronics, particularly for lower flux levels. Even at present, however, the improved monitoring system electronics cannot provide the requisite sensitivity for accurately monitoring and thereby properly and safely indicating the reactor core power levels. Moreover, continued progress in reactor core and fuel design technology, both as incorporated into existing nuclear power plants and as will be implemented in the design of future plants, will result in further reduction or attenuation of the excore neutron fluency levels, which will further compound the present problem of inadequate monitoring sensitivity of excore detecting systems.
Many approaches have been considered to compensate for the reduction in the excore neutron fluency levels and effectively to increase excore detector sensitivity. Essentially all have encountered obstacles which either preclude practical implementation and/or are unsuccessful in achieving the required and intended, increased sensitivity of excore detectors. Repositioning, or realigning of existing excore detectors into greater proximity with the reactor vessel in many instances is not even possible, and, even where possible, typically will result in altering the correlation between the sensed fluency level and the actual level of operation within the reactor vessel. Correction, if possible, requires complex recalibration and testing to assure correct correlation of the detected and the resultant, indicated conditions, relative to actual. Modifications of the excore detector design and/or in the environment, including structural aspects, between the core baffle and the excore detectors, pursued in an effort to increase detector sensitivity, as well introduce the predictable result of altering the neutron fluency levels to which the excore detectors are exposed, with a resultant and correspondingly adverse effect on the accuracy of the indications produced by the neutron flux monitoring system.
A very pronounced material constraint on structural modification of the excore detectors is that for existing reactor vessel installations, the vertical thimbles, or wells, in which the excore neutron detectors are located cannot be modified since they are integral with the plant structure and are inaccessible. Thus in any practical sense, any excore detector structural modifications contemplated for improving sensitivity must still permit the location of the excore detector within the existing detector thimbles, or wells.
Aside from the constraints on structural modifications of the excore detectors so as to maintain compatibility with existing thimbles, or wells, a further critical factor is that any such modified detector structure must still satisfy, and thus not violate, the seismic qualification standards established by the U.S. Nuclear Regulatory Commission. From a practical standpoint, substantial time and expense would be entailed in achieving requalification of a modified detector structure; more importantly, seismic qualification of an excore detector is an essential safety factor, since the detector functions as an integral component of the reactor core-neutron flux monitoring system. Thus, continued viability of excore detectors during a seismic occurrence is essential to maintaining safe levels of operation of the reactor and/or to generating, reliably, an alarm indication when those levels are exceeded.
There are yet further, practical constraints on possible modifications of the excore detectors, beyond the above-noted structural compatibility and seismic qualification requirements. For example, the requisite electrical shielding of the detectors must be maintained, to assure that accurate electronic sensing signals are produced with minimum susceptibility to noise and other sources of electrical interference and to avoid the creation of electrical ground loops. Typically, the excore detector housing (also termed the "outer shield" of the detector) is connected to the electrical ground of the electrical instrumentation system through an associated conductor of a triaxial cable (other leads of which respectively supply high voltage to the detector from the NIS and supply the detector output signal(s) to the NIS). The principle point is that the detector must be isolated from the thimble, or well, within which it is installed and all high voltage, signal and reference (e.g., ground) levels which are supplied to or from the detector must be related solely to the NIS, thereby to minimize noise susceptibility and the creation of ground loops. Numerous other requirements are also imposed, such as the ability of the detector assembly to be readily removed and replaced within the thimble, or well; thus any structurally modified detector still must accommodate the existing lifting and/or other support attachments and mechanisms. Further, allowance must be made for water drainage from the detector assembly, to correct for the typical occurrence of fluid leakage impinging onto and thereby collecting within the thimble assembly, or well, within which the detector is housed. A further practical design consideration is that any modified excore detector structure preferably should maintain the compatibility, or interchangeability, of same for both top-mounted and bottom-mounted installations in corresponding, different reactor vessels.